As new generations of wireless communication devices become smaller and packed with more multi-band functions, designing antenna systems for such devices becomes more challenging. In particular, a communication device with an air interface tends to be affected by use conditions such as the presence of a human hand, a head, a metal object and other interference-causing objects placed in the vicinity of an antenna, resulting in impedance mismatch at the antenna terminal. Designing internal antennas for devices that have partial or complete metallized back covers, such as a metal back cover on a cell phone or Tablet adds an additional parameter that needs to be optimized if good antenna performance is to be maintained. Accordingly, novel antenna design techniques are needed to provide efficient antenna performance for internal antennas when integrated into communication devices that have metallized housings or covers. Ideally, these novel techniques need to have little or no impact on the aesthetics of the industrial design.
As the cellular mobile communications industry transitions from 2G/3G standards to 4G standards the cellular antenna system in the mobile device is required to transition from a one antenna to a two antenna system. This is required to meet the multi-input multi-output (MIMO) architecture used in 4G long term evolution (4G LTE) standard. When other antenna functions in a modem mobile communication device are considered the number of antennas can increase to five. In a typical design engagement for a mobile device where a metal housing is not implemented, considerable time is spent not only designing these five antennas, but also determining optimal placement and orientation to achieve the necessary levels of isolation between the various antennas, as well as correlation coefficient between the two MIMO antennas. Adding a metallized housing to the design process will significantly complicate the antenna system design process.
The MIMO requirement brought about by the 4G LTE standard complicates the antenna system design process due to the addition of the second cellular antenna and the risk of antenna de-tuning of both antennas as a function of the use cases for the mobile device. Use cases may include: hand use, head and hand, or placement of the device on a table or other surface, among others. The complications incurred regarding MIMO antenna system design increase when a metallized housing or cover is considered, due to the direct loading of the metal cover with a user's hand or contact with a surface such as a wooden table, metal file cabinet, or other materials. With cellular communication systems becoming more loaded and capacity constrained, the antenna systems on the mobile side of the communication link are expected to become more efficient to assist in maintaining a level of acceptable network performance. Under-performing mobile devices in regard to the radiated performance of the device will degrade the cellular network, with these under-performing devices requiring more system resources compared to more efficient mobile devices.
Several solutions have been proposed over the years to improve the Total Radiated Power (TRP) and Total Isotropic Sensitivity (TIS) performance of the cellular antenna or to fulfill Specific Absorption Rate (SAR) and Hearing Aid Compatibility (FRC) requirements. Though various passive antenna techniques and topologies have been proposed and developed to improve antenna efficiency for internal applications, they all suffer from the limitation of being optimized for a single use case such as device in user's hand, device against the user's head, or device in free space environment. Implementing a tunable antenna, one where the antenna impedance properties can be modified dynamically, can provide an antenna system that can be optimized for a wider variety of use cases. A common technique for implementing a tunable antenna is to design a tunable capacitor into a passive matching circuit, with the matching circuit located at the feed point of the antenna and used to match the antenna. With tunable antennas implemented for both antennas in a MIMO antenna system inside a mobile device with metal cover or housing, the antennas can be dynamically impedance matched as loading of the metal cover is changed or altered.